In the early days of exhaust-gas cleaning of combustion engines, only the exhaust gases from petrol engines were cleaned with three-way catalysts (TWC). The nitrogen oxides are reduced with the reductive hydrocarbons (HC) and carbon monoxide (CO). For this, the petrol engine is always driven under approximately stoichiometric conditions (λ=1). This cannot always be guaranteed precisely in this way, with the result that the conditions in the exhaust gas always fluctuate around λ=1. In other words, the catalyst is exposed alternately to an oxidative or a reductive gas atmosphere.
For about 15 years, attempts have also been made to aftertreat the exhaust gases from diesel engines with catalysts. The exhaust gas from diesel engines contains carbon monoxide, unburnt hydrocarbons, nitrogen oxides and soot particles as air pollutants. The unburnt hydrocarbons comprise paraffins, olefins, aldehydes and aromatics. Unlike the petrol engine, the diesel engine always runs with an excess of oxygen and thus the catalyst is never exposed to reductive conditions. This has the following consequences:                1. The oxygen storage capacity of the catalyst material does not play the same role as with the TWC.        2. The noble metal particles are not always reduced again to metal of oxidation state 0.        3. The nitrogen oxides cannot be fully reduced when there is an excess of oxygen with the hydrocarbons (HC) present in the exhaust gas and CO.        4. The hydrocarbons and CO can be oxidized both with oxygen and with NOx.        
Diesel exhaust gases are much colder than exhaust gases from petrol engines and contain oxygen in a concentration between 3 and 10 vol.-%, which is why the catalytic activity of the catalyst on average is not always sufficient to oxidize HC and CO. In partial-load operation, the exhaust-gas temperature of a diesel engine lies in the range between 100 and 250° C. and only in full-load operation does it reach a maximum temperature between 550 and 650° C. In contrast, the exhaust-gas temperature of a petrol engine lies between 400 and 450° C. in partial-load operation and, in full load, can rise to up to 1000° C. It is therefore an aim to achieve as low as possible a CO light-off temperature.
In past years, diesel particle filters (DPF) have increasingly been introduced onto the market. These are normally fitted downstream of the DOCS. Soot is collected and oxidized in the DPF. The oxidation of soot is much more possible with NO2 than with oxygen. Thus, the more NO2 is contained in the gas stream after the DOC, the more soot continuously reacts. Thus, there has been a tendency in past years to oxidize as much NO to NO2 as possible in the DOC. But NO2 is an even more toxic gas than NO, with the result that this shift towards increased nitrogen oxide emissions manifests itself in a very negative way. An increasing NO2 concentration due to DOC is also already detectable in cities. Thus, the trend is returning to a limiting of the oxidation of NO to NO2.
Markedly reduced emissions of nitrogen oxides have thus also been prescribed for the Euro VI standard. It will be possible to achieve these either only by means of NOR-trap catalysts or by means of a selective catalytic reduction by means of ammonia.
SCR (selective catalytic reduction) denotes the selective catalytic reduction of nitrogen oxides from exhaust gases of combustion engines and also power stations. Only the nitrogen oxides NO and NO2 (called NOx in general) are selectively reduced with an SCR catalyst, wherein NH3 (ammonia) is usually admixed for the reaction. The closer the NO/NO2 ratio is to 1:1, the more efficiently such an SCR reaction runs, thus a substantial oxidation of NO to NO2 is necessary for this. Only the harmless substances water and nitrogen form as reaction product in the SCR reaction.
The transportation of ammonia in compressed-gas bottles is a safety risk for use in motor vehicles. Therefore precursor compounds of ammonia which are broken down in the exhaust-gas system of the vehicles accompanied by the formation of ammonia are customarily used. For example the use of AdBlue®, which is an approximately 32.5% eutectic solution of urea in water, is known in this connection. Other ammonia sources are for example ammonium carbamate, ammonium formate or urea pellets.
However, a problem is that such an SCR catalyst is difficult to retrofit, or retrofitting is associated with extremely high costs, as many additional components and control systems are necessary. Accordingly, particle filters are today predominantly retrofitted in diesel vehicles without downstream SCR catalyst, wherein excess NO2 that is not required for the particle oxidation leaves the exhaust-gas system and is introduced into the environment. It would thus be advantageous to be able to set the quantity of NO2 produced more precisely.
The oxidation of NO to NO2 takes place in an upstream oxidation catalyst which is thus necessary for an optimum degree of efficiency of the DPF.
The basis of the catalytic exhaust-gas cleaning in a diesel engine is thus clearly the upstream oxidation catalyst which is to have an efficient oxidation action for CO and HC. This is achieved for example by reducing the CO light-off temperature. However, the NO oxidation tendency is to be reduced in order to emit as little NO2 as possible. On the other hand, as far as possible, so much NO2 is to form that a particle filter (without subsequent SCR catalyst) is provided with enough NO2.
It is known in the state of the art (see for example U.S. Pat. No. 5,157,007 A1) that catalysts with TiO2 as support material and V2O5 as catalytically active component have a lower activity with regard to the oxidation of NO to NO2. However, vanadium is toxic and can be introduced into the environment via the exhaust-gas system. In addition, vanadium also reduces the activity vis-à-vis the CO oxidation and thus is not desired for this reason also.
In the publication SAE 2005/01-0476 (Rhodia), it is clear that above all support materials with smaller interactions with Pt(II), e.g. aluminium oxide and zirconium oxide, make possible very low light-off temperatures for the oxidation of CO. Because of the larger BET surface area of aluminium, aluminium oxide is preferably used for DOC applications.
One way of reducing the light-off temperature for CO as much as possible can be found in the patent application EP 706817 A1 from Umicore. EP 706817 A1 describes a DOC catalyst with Pt on an Al/Si mixed oxide (in the best case 5% Si).
The further development using an H+ and Na+ zeolite is disclosed in EP 800856 B1, where light-off temperatures of approximately 150° C. for CO are already achieved.
A further improvement is described in EP 1129764 B1, where very finely distributed Pt particles with an average oxidation state of the Pt<2.5 form by a calcining by means of injection into a flame. It is to be borne in mind that combustion exhaust gases can contain a wide variety of components, such as CO, nitrogen oxides and residual hydrocarbons. In addition, combustion exhaust gases can also contain different quantities of oxygen depending on the guidance of the combustion. The gas mixture can thus be reductive or oxidative.
Although the injection of a platinum precursor into a flame results in a catalyst that has a good activity with regard to a CO oxidation, the oxidation activity with regard to the oxidation of NO to NO2 cannot be controlled with this method. Thus, there is still a need for catalysts with as low as possible a light-off temperature for CO and, at the same time, a low activity and selectivity for the oxidation of NO to NO2.
The object of the present invention was therefore to provide such catalysts.